1. Field of the Invention
The present invention relates to a power transmission apparatus for a four-wheel-drive vehicle.
2. Description of the Related Art
In a conventional transverse engine (an engine wherein the pistons are arranged perpendicular to the body of the vehicle), as disclosed in Japanese Patent Provisional Publication No. 60-18656, one-shaft type which transmits driving force from a first shaft is well known. In this type of engine, however, the first shaft is provided at a rather high position with respect to the vehicle height. Accordingly, a problem arises in a layout where a floor tunnel covering a propeller shaft connecting to a second shaft is also provided at the high position.
In light of the above drawback, a two-shaft-type four-wheel-drive power transmission apparatus which can variably control division of drive torque for the rear wheels is suggested. According to Japanese Patent Provisional Publication No. 62-50235, it is disclosed that the front wheels are constantly driven by the transverse engine in a vehicle front and a clutch is provided in the rear-wheel drive transmission system.
The construction is described with reference to the figures. FIG. 10 is a partial plane based on the figure disclosed in the above Patent Provisional Publication. In the figure, a clutch means is comprised of an oil clutch and controlled by oil pressure. A transmission gear 2 is fixed so as to be arranged substantially in-line with respect to the transverse engine 1 indicated by a two-dot chain line in the figure. A final ring gear 5 is engaged with a pinion gear 4 (indicated by a broken line) which is an output shaft of the transmission gear 2. The final ring gear 5 is fixed on a peripheral surface portion of a front differential gear 6. A first axle shaft 7 for driving the front wheels through the front differential gear 6 is rotatably supported by bearing means indicated by the notations .DELTA. and .gradient.. The first axle shaft 7 connects with the front drive shafts 8 which constantly drive the front wheels through a universal coupling. With the above-described construction, a system called a FF drive (Front-engine Front-drive) system which constantly drives the front wheels is formed.
In order to variably control division of the drive torque of the rear wheels, a transfer gear 25 fixed to the transfer shaft 12, which is arranged in parallel to the first axle shaft 7, is engaged with the final ring gear 5 at the position which is the opposite to the pinion gear 4. Accordingly, the torque of the engine 1 is transmitted to the transfer shaft 12. On the other hand, the drive-force transmission to a propeller shaft 15 is performed by transforming the direction of the driving force approximately 90 degrees by a front hypoid pinion gear 14 which is engaged with a front hypoid ring gear 130 fixed on the transfer shaft 12, and the drive-force transmission is further performed to each of rear drive shafts 22 through a rear differential gear 19 for the rear-wheel drive.
The power transmission apparatus of the four-wheel drive having the following constitution is also suggested. A clutch 16 which is operated by oil pressure is provided on the transfer shaft 12. An oil pump 101 which generates a predetermined oil pressure and a motor 102 are connected to the clutch 16 and controlled by a controller 100 according to vehicle speed, four-wheel-drive selection, and accelerator opening. Accordingly, in addition to the front wheels, the rear wheels are actively controlled.
In the conventional two-shaft type four-wheel drive (the first axle shaft 7 and transfer shaft 12 are provided), in order to lower the floor, driving-force transmission to the propeller shaft 15 is performed by transforming the driving-force direction approximately 90 degree by the front hypoid pinion gear 14 which is engaged with the front hypoid ring gear 130 fixed on the transfer shaft 12. The power transmission to each of the rear drive shafts is further performed through the rear differential gear for the rear-wheel drive. In this construction, the front hypoid ring gear 14 has a reversing tooth surface with respect to the rear hypoid ring gear 18.
FIG. 11 is an external view showing the state where the rear hypoid ring gear 18 and the hypoid pinion gear 17 which are incorporated in the rear differential gear 19 are engaged, and it is shown in order to explain the above reverse-rotation tooth surface. As shown in FIG. 11, a tooth portion 18t of the rear hypoid ring gear 18 is a helical gear which is twisted to the right direction. The tooth portion 18t is driven to the normal-rotation direction indicated by a solid line and the reverse-rotation direction indicated by a broken line with a center line CL3 indicated by a dashed line. A tooth portion 17t of the rear hypoid pinion gear 17 is also a helical gear which is twisted to the left direction. The tooth portion 17t of the rear hypoid pinion gear 17 has a center line CL2 which is shifted in the distance F from the center line CL3 of the rear hypoid ring gear 18. When the pinion shaft 17a fixing the rear hypoid pinion gear 17 on its one end is driven to either the normal-rotation direction or the reverse-rotation direction, the tooth portion 17t is smoothly abutted against the tooth portion 18t and power transmission is performed. Accordingly, the normal-direction tooth surface is used for the tooth portions 17t and 18t because it is advantageous for the load in the normal-rotation direction which is caused by high frequency of usage.
That is, as shown in FIG. 12 which is, a vertical sectional view of FIG. 11 at the XII--XII line the tooth portion 18t of the hypoid ring gear 18 forms a right tooth surface 18c and left tooth surface 18b comprising a part of an involute curve. The inclined angle of the left tooth surface 18b with respect to the base of the hypoid ring gear 18 is set to a value which is smaller than that of the right tooth surface 18c. Both the right tooth surface 17b and the left tooth surface 17c of the tooth portion 17t of the hypoid pinion gear 17 are abutting against the right tooth surface 18b and right tooth surface 18c, and the driving force of the hypoid pinion gear 17 is transmitted. When the hypoid pinion gear 17 is forwarded (rotated in the normal-rotation direction), the left tooth surface 18b of the rear hypoid pinion gear 18 and the right tooth surface 17b of the rear hypoid pinion gear 17 are abutted, while an abutting force K1 is received at the left tooth surface 18b. On the other hand, when the hypoid pinion gear 17 is reversed, the right-tooth surface 18c and left-tooth surface 17c are contact, and an abutting force K2 is received at the right tooth surface 18c. As the left tooth surface 18b has a smaller inclined angle, it can reserve more strength. Accordingly, the left tooth surface 18b is used for the forwarding side, while the right tooth surface 18c having a large inclined angle is used for the engine-brake side for reversing.
On the other hand, the tooth surface of the front hypoid ring gear 130 (refer to FIG. 10) cannot transmit the torque of the transfer shaft 12 to the propeller shaft 15 which rotates in the normal-rotation direction, if it is arranged so as to face the same direction as that of the rear hypoid ring gear 18. Accordingly, the rear hypoid ring gear 18 or an equivalent gear cannot be used. Therefore, the hypoid ring gear 130 using the right tooth surface 18c for the normal-rotation direction is processed, and this is used for the front hypoid ring gear and called as a hypoid ring gear having a reverse tooth surface.
However, in a gear transmission type such as the above two-shaft type, the direction of rotation of the transfer shaft and that of the second shaft are opposite to each other. Accordingly, since the ring gear and pinion gear of the bevel gear comprising a gear set both face in the same direction, say the right side, one becomes a normal-rotation tooth surface, while the other is a reverse-rotation tooth surface.
Accordingly, a three-shaft type is suggested. That is, an idle shaft is provided between the first shaft and transmission shaft. The direction of rotation of the transfer shaft is arranged to be the same as that of the second shaft, and both use the normal-rotation tooth surface.
For example, a diagram of FIG. 13 is disclosed in Japanese Patent Provisional Publication No. 63-287632. In this figure, the idle shaft 11 is provided between the first axle shaft 7 and transfer shaft 12. In this way, it is arranged so that only the normal-rotation tooth surface can be used.